The present invention is generally related to a wireless access network utilizing ATM in fixed network infrastructure, and more particularly to using the header fields of an ATM cell in an efficient way from the point of view of the data transfer, associated with the said wireless connection segment.
The ATM (Asynchronous Transfer Mode) is strongly offered as a candidate for the fast data transfer system of the future e.g. in the B-ISDN networks (Broadband Integrated Services Digital Network) and in the mutual communication of information transfer devices in general. The network consists of nodes and terminal devices and the connections between them. In the ATM network, information is transported in digital form as cells each of which comprises a so called payload of 48 bytes and a header part of 5 bytes. In order to keep the amount of the header information as small as possible, no complete information on routing between the transmitting and receiving device is included in the headers, but only information on the virtual path and channel where the data in question is transported. The nodes of the network include the required routing information, according to which the corresponding identifiers of the virtual path and channel are interpreted as addresses to the next node, respectively.
Use of the cell header is strictly specified in the specifications of the ATM concept. The header includes a Virtual Path Identifier (VPI) of 8 through 12 bits, a Virtual Channel Identifier (VCI) of 16 bits, a Generic Flow Control (GFC) of 0 through 4 bits, 3 bits to express the type of the payload and one bit to express the priority class of the cell and 8 bits including an error correction code calculated according to the rest of the cell header.
Traditionally, ATM connections have been considered as fixed wired or optical fiber connections. It can be seen, however, that in the data transfer solutions of the future, the terminal devices must be provided with the same mobility and independency from the fixed connections that the mobile phones of cellular networks already have. Because of its functionality and flexible variability, the wireless access network of cellular type is an obvious architecture also in the future. A typical cellular network comprises several Base Transceiver Stations (BTS) that are usually connected over the Base Station Controller (BSC) to the
Mobile Switching Center (MSC). A wide cellular network has numerous Mobile Switching Centers and their subordinated Base Transceiver Stations and Base Station Controllers. The coverage area of each base station, in other words, the geographical area, where the terminal devices can have a radio connection with the said base station, is called a cell. In order to avoid confusions, however, the word cell refers in this patent application only to the unit of the data to be transported in accordance with the specifications of the ATM system, the length thereof being 53 bytes.
The part of the data transfer network where wireless connection is essential for the data transport is called a Wireless Access System (WAS), or Radio Access System when radio communications are used. When connecting a cellular radio network with the wired connection ATM network, the radio access system, generally specified, comprises the data transfer segments from the terminal device to the point, from where the transport is forwarded without any changes to the general ATM specifications. The Wireless Access System generally comprises e.g. terminal devices, base stations and a base station controller, the last mentioned having a wired connection with the general ATM network. Because of the mobility of the terminal devices and the character of the data to be transferred, it is necessary to transfer within the radio access system additional information that is not separately specified in the cell specifications of the ATM system. The additional information is e.g. associated with the macro-diversity, in other words, routing the connection simultaneously along several parallel paths, with the handover function for changing the base station and in case of a telephone connection with packing of speech data. The problem is, how this data will be transported within the radio access system in the most efficient way without unnecessary delays and waste of resources. In addition, the macro-diversity and handover functions require some kind of synchronizing between the cells transferred over different base stations, because the ATM system as such does not include any mechanisms for correcting errors arising from cell losses or from changes in their mutual sequence. Errors on the cell level have the influence that the data structure determined on some higher layer and comprising several cells (PDU, Protocol Data Unit) is noticed to be incorrect and will be rejected, whereupon maybe lots of correct cells must be retransmitted.
Known from the patent publication EP 366 342 (AT&T) there is a procedure, according to which the data is transported in a radio connected cellular network as cells, and in the header part of each cell there is an unchanging part that remains unchanged independently from the changes in the routing, and a changing part, the contents of which will be changed in connection with the base station handover or some other change of the routing. It is stated in the publication that the arrangement can make the base station handover easier, but the relief is aimed only at the routing definition. A same kind of procedure is presented in the patent application EP 577 959 (Roke Manor Research Ltd.), where it is especially aimed at the ATM network. There the unchanging part of the cell header is the so called VCI field (Virtual Channel Identifier) and the changing part is the so called VPI field (Virtual Path Identifier).
Known from the article "LAN Emulation on an ATM Network" of Hong Linh Truong, IEEE Communications Magazine, May 1995, there is a short addressing method, according to which an address space of only 12 bits can be used in the local area network emulation. It is proposed in the publication that the number of 4096 potential addresses of 12 bits is sufficient for the needs of a small local network, so that the VPI field of the ATM cell in its long form can be used as an address. The short addressing data alone is, however, insufficient for transporting the above mentioned additional data. It has been proposed that for transporting the additional data, the payload part of the ATM cells would be used, but as thereby the share of the actual data to be transferred is decreased, the efficiency of the method suffers. It has also been proposed that in the radio connection two different channels would be used for transporting the additional data and the actual data to be transferred, respectively. This spends the resources of the system, too, and in addition, it is not possible to transfer accurately the data addressed specifically to a certain ATM cell.
Known from the article "BAHAMA: A Broadband Ad-Hoc Wireless ATM Local-Area Network" of K.Y.Eng et al, Proc.ICC'95, Jun., 18-22, 1995, Seattle, there is a procedure, where the GFC field included in the header part of the ATM cells is used for implementing the sequence numbering applying to cell. It is the intention to help synchronizing and combining of the cell streams coming along two parallel channels to a certain combining point. The numbering of the cells is especially aimed at an unequivocal identification of the cells, so that the cells by the combining of cell streams won't be doubled or lost and their sequence will remain unchanged. The numbering alone is, however, insufficient for transporting all the necessary additional data and through the four bits of the GFC field, only a very limited amount of information can be transported, anyway. It is one object of the present invention to provide a method where the data transfer within the radio access system in connection with the ATM network is efficiently executed. It is another object of the present invention to provide a method for improving the use of the base station handover and macro-diversity operations in this kind of a radio access system. It is a further object of the invention to provide an equipment for implementing this kind of a method.